A simple variable frequency sinewave audio generator.

UNIX macOS
1

Hi all...

Well I have not been inactive but working out how to make OSX 10.14.x command line audio player have a variable sample rate.
This is a back door as afplay does not have a sample rate flag unlike aplay for ALSA, in Linux flavours.
This is a DEMO only but a derivative of it will be used in a kids level, variable, AUDIO Function Generator from around 50HZ to 20KHz for pure audio use.

The other limitation is that the shell script must adhere to full POSIX compliance.
I use dash in the script but it should work on the majority of shells.

How does it work?
Well it creates the file everytime you run the script in the following order:
First the frequency '$1' is converted to the big endian word, hex, sample rate.
Next the bytes are extracted and converted to octal.
Finally these octal bytes are reversed and a string created in the correct order.

Now the 'WAV' header is built up using octal values with the 'RATE' placed in the middle.
After that the 'DATA' is added which makes the whole file 65580 bytes in size.

Although this was designed purely for OSX's command line audio player it also works with the ALSA's one too.

USAGE: [./]VFO.sh <Integer FREQUENCY value from 250 to 6000>
As it is a DEMO there is only limited error detection!

#/usr/local/bin/dash
# #!/bin/sh
#
# USAGE" [./]VFO.sh <Integer FREQUENCY value from 250 to 6000>

FREQ=$1
if [ "${FREQ}" = "" ]
then
    FREQ=1000
fi

if [ ${FREQ} -lt 250 ] || [ ${FREQ} -gt 6000 ]
then
    FREQ=1000
fi

# Get individual little endian words, convert to bytes, reverse and convert to octal.
RATE=$( printf "%04x" $(( FREQ * 8 )) )
BYTE2='\'$( printf "%03o" "0x${RATE%??}" )
BYTE1='\'$( printf "%03o" "0x${RATE#??}" )
# The RATE string to be inserted into the header.
RATE="${BYTE1}${BYTE2}"'\000\000'"${BYTE1}${BYTE2}"

# Create the wav header, 44 bytes in size.
: > /tmp/sine.wav
printf "%b" "\122\111\106\106\044\000\001\000\127\101\126\105\146\155\164\040\020\000\000\000\001\000\001\000" >> /tmp/sine.wav
printf "%b" "${RATE}" >> /tmp/sine.wav
printf "%b" "\000\000\001\000\010\000\144\141\164\141\000\000\001\000" >> /tmp/sine.wav

# Add the sinewave data, 65536 bytes in size.
DATA="Oq~qO- -"
COUNTER=0
while [ ${COUNTER} -le 12 ]
do
    DATA=${DATA}${DATA}
    COUNTER=$(( COUNTER + 1 ))
done
# Entire filesize 65580 bytes, 8+ seconds long for 1000[Hz].
printf "%b" "${DATA}" >> /tmp/sine.wav

# OSX command line player.
afplay /tmp/sine.wav > /dev/null 2>&1

# ALSA command line player.
aplay /tmp/sine.wav > /dev/null 2>&1

Enjoy...

Bazza.

2 Likes
2

A suggestion for more efficiency:

# Open the file for writing via descriptor 4
exec 4> /tmp/sine.wav
{
printf "%b" "\122\111\106\106\044\000\001\000\127\101\126\105\146\155\164\040\020\000\000\000\001\000\001\000"
printf "%b" "${RATE}"
printf "%b" "\000\000\001\000\010\000\144\141\164\141\000\000\001\000"
} >&4
# Add the sinewave data, 65536 bytes in size.
DATA="Oq~qO- -"
COUNTER=0
while [ ${COUNTER} -le 12 ]
do
    printf "%b" "${DATA}"
    COUNTER=$(( COUNTER + 1 ))
done >&4
# Entire filesize 65580 bytes, 8+ seconds long for 1000[Hz]
2 Likes
3

Thanks MadeInGermany, I will try this out on my main code.

I limited the DEMO to 6000Hz because it kept the BYTE manipulation easy to understand using only one little endian word per sample and byterate instead of the two little endian words for both. Also 8 bits unsigned per sample, and mono eliminates the need for other sections to be altered.

As an addendum the WAV file format:

Microsoft WAVE soundfile format